Originally Posted by dkersten

Here is the test we need to settle this once and for all: Start with stock (2005-2010 3v 4.6). Tune it to get the most power. Dyno, record numbers. Install LT headers. Make changes to tune only for O2 sensors so you don't throw a code. Now dyno again and compare.

My money is on the two dynos being nearly identical with maybe a slight change in the torque curve and peak points for hp and torque.